Two-component epoxy resin systems on a water-dilutable basis are known in the industry. They are used e.g. for prevention of corrosion. The properties of the coating agent can be influenced via the choice of epoxy resin or the choice of the amine cross-linking agent. The general principles of this method are described in the article by C. Godau in I-Lack 4.90, Volume 58, page 134. The advantages are said to be good elasticity and good adhesion of the wet film. The article refers to the difficulties of efficient film formation, which depends on the drying conditions and the pot life of the resin mixture. It is also stated that the stability of the dispersion is adversely influenced e.g. by pigments with a high content of salt-forming, i.e. ionic, substances.
EP-A 0 176 192 describes a two or three-component material consisting of an epoxy resin, a polymer produced by radical polymerisation of olefinically unsaturated monomers, and a binder containing groups which react with epoxy groups. Aqueous dispersions are produced via hydrophilic groups or by using emulsifiers.
DE-OS 38 25 754 describes two-component epoxy-amine systems which can be diluted with water. The water content is below 20%, and consequently the coating agent cannot be described as aqueous. The crosslinking components mentioned are aliphatic di-primary diamines containing 2 to 12 carbon atoms. The film-forming constituents are stated to be polyglycidyl compounds based on bisphenols or polyols, together with a binder based on OH-containing polyacrylates or polyethers. A disadvantage of these coating agents is that the properties of the film are impaired when the water content rises. Compositions of this kind are not particularly suitable as regards protection of the environment.
EP-A 0 319 841 describes two-component aqueous epoxy-amine coating agents. One component consists of an aqueous dispersion of epoxy resins whereas the other component consists of a special urethane oligomer with terminal amino-functional groups, between 25 and 40% of which are neutralised with acid. Up to 20% of organic solvents are also added, to obtain a sufficiently level film. Special process steps are disclosed for manufacturing the amino functional polyurethane oligomer.
One general difficulty of the known conventional 2-K aqueous coating agents based on epoxy resin and polyamine is that the aqueous solutions must be stable. When ionic groups are used, the stability of the aqueous dispersion mixture is often unsatisfactory. Another disadvantage of coating agents based on ionic dispersions is that the resulting surfaces are non-uniform, e.g. with pin holes after liquefying. In addition, the known 2K epoxy-amine coating agents are unsuitable for plastics lacquering, since they are too inelastic after crosslinking.
The aim of the invention is to provide aqueous epoxy-amine coating agents which are stable individually or in individual components and yield stable dispersions when mixed and produce smooth, fault-free surfaces after application. Another aim is to obtain coatings with good adhesion to the substrate and subsequent layers with high elasticity.
To this end, the coating agent comprises an aqueous epoxy resin-containing film-forming binder and a crosslinking component based on an aqueous or water-dilutable polyamine, and either the epoxy resin component or the polyamine component or both components together contain 5 to 80 wt. %, relative to the total epoxy resin and polyamine, each considered as resin solids, of a polyurethane resin in water-soluble or water-dispersible form. The aqueous polyurethane resin contains substantially no reactive groups used for crosslinking; it can be stabilised by ionic or non-ionic substances. The coating agent can also contain pigments, fillers and/or other conventional additives in the lacquer industry.
According to the invention, water-dilutable binders and crosslinking agents are used. "Water-dilutable" means convertible into an aqueous form, either a dispersion, emulsion or molecular dissolved form. The solvent in the coating agent is mainly water.
The binder components in the coating agents according to the invention can be conventional di- or polyepoxides. These binder components are film-forming epoxy resins in the form of an aqueous dispersion or water-dilutable resin. The polyepoxides can e.g. be polyglycidyl ethers based on aliphatic or aromatic diols, such as bisphenol A, bisphenol F, novolaks or polyalkylene glycols which are commercially available and well-known to the skilled addressee.